Metal-insulator transition induced by electronic and structural modulations in oxygen-deficient perovskite-type TbBaCo2O5.5

Abstract

The metal?insulator (M?I) transition at about 340 K (TMI) in TbBaCo2O5.5 has been investigated by X-ray absorption spectra near Tb-LIII, Ba-LII, and Co-K edges. In the metallic and insulating phases, the high-density unoccupied bands of Tb-6d? and Ba-6d? occupy just above a Fermi level (EF), while the positive charge-transfer Tb-5dnLm and Ba-5dnLm bands are just below EF. The narrow positive charge-transfer Co-3d7(eg)L band is just below EF, where L represents a hole induced in the valence band of O-2p. It is considered that the metallic conductivity above TMI results from the (p?d) hybridized holes in the top region of the O-2p valence band and the hopping electrons in the bottom of the O-2p conduction band. The spin-state transition for Co ions in the CoO6 octahedra and the CoO5 pyramids does not occur at TMI. The local structural distortion induced around the Ba ions, but not around the Tb and Co ions, mainly affects the electronic configuration interaction between the Ba-5d and the O-2p orbitals in BaO. (? 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)